I was looking through all questions concerning the usage of そうです。.Now, in Course 4 Lesson 2 the way to use it is explained. However, only for い-Adjectives.Someone asked the question under the title "How do you use SOU DESU with -NA adjectives?", however the usage was only explained for the positive form.

しずかそうです。べんりそうです。...But what if I want to say that someones doesn't appears to be a な-Adjective? My guess would be using じゃない。しずかじゃなそうです。 - Does not appear to be quiet.べんりじゃなそうです。 - Does not appear to be convenient